276 The Ohio Naturalist. [Vol. XIV, No. 5, 



air. This explains the reduction of nitrate in water-logged surface 

 soil and the tendency to denitrification in heavy compact soils as 

 compared with the more open sandy loams. 



Drainage, liming, and thorough tillage, greatly lessen the 

 danger from denitrification by improving the circulation of air in 

 the soil. 



The Symbiotic Root Tubercle Nitrogen 

 Fixing Bacteria. 



Much has been written regarding the relation of legumes to 

 the tubercle forming bacteria that grow upon their roots. But 

 because of their importance to a pennanent system of agriculture, 

 it is well to call attention to them in this short review of the soil 

 bacteria. 



These root tubercle bacteria (Bacillus radiciola) are parasites. 

 They require carbohydrate material and are unable to manufacture 

 it from the elements of carbon, hydrogen, and oxygen; conse- 

 quently they derive it by growing upon the roots of leguminous 

 plants. The agricultural plants included under the term legumes 

 are: alfalfa or lucerne; clover; melilotus or sweet clover; peas; 

 beans; and vetches. 



The bacteria can enter the roots of legumes when the latter 

 are in a weakened condition, such a state resulting when the 

 nitrogen supply of the soil is deficient. In a weakened state, 

 they have slight power of resistence, and the nodule bacteria, 

 seeking carbohydrate material, gain enterance to the root through 

 the tip of the root hairs. The bacteria may possibly secrete an 

 enzyme which dissolves the substance of the tip of the root. 

 After they have entered the root, the bacteria cause excessive 

 reproduction of the plant tissue about it, which results in the 

 formation of the tubercles. The bacteria are not found in all 

 parts of the plants, but are confined to the nodules and rootlets. 

 The presence of bacteria upon seeds results from the contam- 

 ination of the seed with soil. 



The symbiotic bacteria, developing in the nodules, absorb 

 nitrogen from the air circulating in the porcspaces of the soil. 

 The nitrogen absorbed by these bacteria becomes immediately 

 available to the plant. Soil fertility, however, is only increased 

 when these plants become dead and have passed through the 

 cycle of decomposition (humus, ammonia, nitrous salts, nitric 

 salts) . 



Although past attempts to develop these bacteria to grow upon 

 non leguminous plants have been unsuccessful, it may yet be 

 possible to do so. 



